Method of preparingiron-based components

ABSTRACT

The present invention concerns a process for the preparation of high density green compacts comprising the steps of providing an iron-based powder essentially free from fine particles; optionally mixing said powder with graphite and other additives; uniaxially compacting the powder in a die at a compaction pressure of at least about 800 MPa and ejecting the green body. The invention also concerns the powder used in the method.

FIELD OF THE INVENTION

The present invention relates to metal powder compositions useful withinthe powder metallurgical industry. More specifically the inventionconcerns a method for the preparation of components having high densityby using these compositions.

There are several advantages by using powder metallurgical methods forproducing structural parts compared with conventional matching processesof full dense steel. Thus, the energy consumption is much lower and thematerial utilisation is much higher. Another important factor in favourof the powder metallurgical route is that components with net shape ornear net shape can be produced directly after the sintering processwithout costly shaping processes such as turning, milling, boring orgrinding. However, normally a full dense steel material has superiormechanical properties compared with PM components. This is mainly due tothe occurrence of porosity in the PM components. Therefore, the strivehas been to increase the density of PM components in order to reachvalues as close as possible to the density value of a full dense steel.

Among the methods used in order to reach higher density of PM componentsthe powder forging process has the advantage that full dense componentsmay be obtained. The process is however costly and is utilised mainlyfor mass production of heavier components, such as connection rods. Fulldense materials can also be obtained by elevated pressures at hightemperatures, such as in hot isostatic pressing, HIP, but also thismethod is costly.

By using warm compaction, a process where the compaction is performed atan elevated temperature, typically at 120 to 250° C., the density can beincreased with about 0.2 g/cm³, which results in a considerableimprovement of the mechanical properties. A disadvantage is however thatthe warm compaction method involves additional investment andprocessing. Other processes, such as double pressing, double sintering,sintering at elevated temperatures etc, may further increase thedensity. Also these methods will add further production costs hencereducing the overall cost effectiveness.

In order to expand the market for powder metallurgical components andutilise the advantages with the powder metallurgical technique there isthus a need for a simple, less expensive method of achieving highdensity compacts with improved static and dynamic mechanical strength.

SUMMARY OF THE INVENTION

It has now been found that high density components can be obtained byusing high compaction pressures in combination with coarse powders. Inview of the general knowledge, that conventionally used powders, i.e.powders including fine particles, cannot be compacted to high densitieswithout problems with e.g. damaged or deteriorated surfaces of thecompacts this finding is quite unexpected. Specifically, the methodaccording to the present invention includes the steps of providing aniron-based powder essentially free from fine particles; optionallymixing said powder with graphite and other additives; uniaxiallycompacting the powder in a die at high pressure and ejecting the greenbody, which may subsequently be sintered.

DETAILED DESCRIPTION OF THE INVENTION

The term “high density” is intended to mean compacts having a density ofabout at least 7.3 g/cm³. Components having lower densities can ofcourse also be produced but are believed to be of less interest.

The iron-based powder according to the present invention includes pureiron powder such as atomised iron powder, sponge iron powder, reducediron powder; partially diffusion-alloyed steel powder; and completelyalloyed steel powder. The partially diffusion-alloyed steel powder ispreferably a steel powder alloyed partially with one or more of Cu, Ni,and Mo. The completely alloyed steel powder is preferably a steel powderalloyed with Mn, Cu, Ni, Cr, Mo, V, Co, W, Nb, Ti, Al, P, S and B. Alsostainless steel powders are of interest.

As regards the particle shape it is preferred that the particles have anirregular form as is obtained by water atomisation. Also sponge ironpowders having irregularly shaped particles may be of interest.

A critical feature of the invention is that the powder used have coarseparticles i.e. the powder is essentially without fine particles. Theterm “essentially without fine particles” is intended to mean that lessthan about 5% of the powder particles have a size below 45 μm asmeasured by the method described in SS-EN 24 497. So far the mostinteresting results have been achieved with powders essentiallyconsisting of particles above about 106 μm and particularly above about212 μm. The term “essentially consists” is intended to mean that atleast 50%, preferably at least 60%, and most preferably at least 70% ofthe particles have a particle size above 106 and 212 μm, respectively.The maximum particle size may be about 2 mm. The. particle sizedistribution for iron-based powders used at PM manufacturing is normallydistributed with a gaussian distribution with a average particlediameter in the region of 30 to 100 μm and about 10-30% less than 45 μm.Iron based powders essentially free from fine particles may be obtainedby removing the finer fractions of the powder or by manufacturing apowder having the desired particle size distribution.

The influence of particle size distribution and the influence ofparticle shape on the compaction properties and properties of thecompacted body have been subjected to intense studies. Thus the U.S.Pat. No. 5,594,186 reveals a method of producing PM components with adensity higher than 95% of theoretical density by utilisingsubstantially linear, acicular metal particles having a triangular crosssection. Such particles are suitably produced by a machining or millingprocess.

Powders having coarse particles are also used for the manufacture ofsoft magnetic components. Thus the U.S. Pat. No. 6,309,748 discloses aferromagnetic powder, the particles of which have a diameter sizebetween 40 and 600 μm. In contrast to iron based powder particlesaccording to the present invention, these powder particles are providedwith a coating.

In the U.S. Pat. No. 4,190,441 a powder composition for production ofsintered soft magnetic components is disclosed. In this patent the ironpowder includes particles with less than 5% exceeding 417 μm, and lessthan about 20% of the powder particles have a size less than 147 μm.This patent teaches that, because of the very low content of particlesless than 147 μm, the mechanical properties of components manufacturedfrom this coarse, highly pure powder are very low. Furthermore thepatent teaches that if higher strength is desired, it is not possible toincrease the content of particles having a size less than 147 μm withoutsimultaneously deteriorating the soft magnetic properties. Thereforethis powder is mixed with specific amounts of ferrophosphorus. Graphitewhich may be used in the compositions according to the present inventionis not mentioned in this patent and besides the presence of graphitewould deteriorate the magnetic properties.

Powder mixtures including coarse particles are also disclosed in theU.S. Pat. No. 5,225,459 ( EP 554 009) which also concerns powdermixtures for the preparation of soft magnetic components. Nor do thesepowder mixtures include graphite.

Within the field of powder forging it is furthermore known thatpre-alloyed iron-based powders with coarse particles can be used. TheU.S. Pat. No. 3,901,661 discloses such powders. This patent disclosesthat a lubricant may be included and specifically that the amount oflubricant should be 1% by weight (example 1). If the powders accordingto the present invention were mixed with such a high amount of lubricantit would however not be possible to achieve the high densities.

In order to obtain compacts having satisfactory mechanical sinteredproperties of the sintered part according to the present invention it isnecessary to add certain amounts of graphite to the powder mixture to becompacted. Thus graphite in amounts between 0.1-1, preferably 0.2-1.0and most preferably 0.2-0.8% by weight of the total mixture to becompacted could be added before the compaction.

Other additives may be added to the iron-based powder before compaction,such as alloying elements comprising Mn, Cu, Ni, Cr, Mo, V, Co, W, Nb,Ti, Al, P, S, and B. These alloying elements may be added in amounts upto 10% by weight. Further additives are machinability enhancingcompounds, hard phase material and flow agents.

The iron-base powder may also be combined with a lubricant before it istransferred to the die (internal lubrication). The lubricant is added tominimize friction between the metal power particles and between theparticles and the die during a compaction, or pressing, step. Examplesof suitable lubricants are e.g. stearates, waxes, fatty acids andderivatives thereof, oligomers, polymers and other organic substanceswith lubricating effect. The lubricants are preferably added in the formof particles but may also be bonded and/or coated to the particles.According to the present invention the amount of lubricant added to theiron-based powder may vary between 0.05 and 0.6%, preferably between0.1-0.5% by weight of the mixture.

The method according to the invention may also be performed with the useof external lubrication (die wall lubrication) where the walls of thedie are provided with a lubricant before the compaction is performed. Acombination of external and internal lubrication may also be used.

The term “at high compaction pressure” is intended to mean at pressuresof about at least 800 MPa. More interesting results are obtained withhigher pressures such as pressures above 900, preferably above 1000,more preferably above 1100 MPa.

Conventional compaction at high pressures, i.e. pressures above about800 MPa with conventionally used powders including finer particles, inadmixture with low amounts of lubricants (less than 0.6% by weight) aregenerally considered unsuitable due to the high forces required in orderto eject the compacts from the die, the accompanying high wear of thedie and the fact that the surfaces of the components lend to be lessshiny or deteriorated. By using the powders according to the presentinvention it has unexpectedly been found that the ejection force isreduced at high pressures, about 1000 MPa, and that components havingacceptable or eaten perfect surfaces may be obtained also when die walllubrication is not used.

The compaction may be performed with standard equipment, which meansthat the new method may be performed without expensive investments. Thecompaction is performed uniaxially in a single step at ambient orelevated temperature. Alternatively the compaction may be performed withthe aid of a percussion machine (Model HYP 35-4 from Hydropulsor) asdescribed in patent publication WO 02/38315.

The sintering may be performed at temperatures normally used within thePM field, e.g. at standard temperature between 1080 and 1160° C. or athigher temperatures above 1160° C. and in conventionally usedatmospheres.

Other treatments of the green or sintered component may as well beapplied, such as machining, case hardening, surface densification orother methods used in PM technology.

In brief the advantages obtained by using the method according to thepresent invention are that high density green compacts can be costeffectively produced. The new method also permits production of highercomponents which are difficult to produce by using the conventionaltechnique. Additionally standard compaction equipment can be used forproducing high density compacts having acceptable or even perfectsurface finish.

Examples of products which suitably can be manufactured by the newmethod are connecting rods, gears and other structural parts subjectedto high loads. By using stainless steel powders flanges are of specialinterest.

The invention is further illustrated by the following examples.

EXAMPLE 1

Two different iron-based powder compositions according to the presentinvention were compared with a standard iron-based powder composition.All three compositions were produced with Astaloy Mo available fromHöganäs AB, Sweden. 0.2% by weight of graphite and 0.4% by weight of alubricant (Kenolube™) were added to the compositions. In one of theiron-based powder compositions according to the invention, particles ofthe Astaloy Mo with a diameter less than 45 μm were removed and in theother composition according to the invention particles of Astaloy Moless than 212 μm were removed. The compaction was performed at ambienttemperature and in standard equipment. As can be seen from FIG. 1-1 aclear density increase at all compaction pressures is obtained with thepowder having a particle size above 212 μm.

FIG. 1-2 shows that in order to obtain components without deterioratedsurfaces the most important factor is the reduction or elimination ofthe smallest particles, i.e. particles below 45 μm. Furthermore fromthis figure it can be seen that the force needed for ejection of thecompacts produced by the iron based powder composition without particlesless than 212 μm was considerably reduced compared with the ejectionforce needed for compacts produced from the standard iron-based powdercomposition having about 20% of the particles less than 45 μm. Theejection force needed for compacts produced from the iron-based powdercomposition according to the invention without particles less than 45 μmis also reduced in comparison with the standard powder.

A noticeable phenomenon is that the ejection force for compacts producedaccording to the present invention decreases with the increasingejection pressure whereas the opposite is valid for the standardcomposition.

It was also observed that the compacts obtained when the standard powderwas compacted at a pressure above 700 MPa had deteriorated surfaces andwere accordingly not acceptable. The compacts, which were obtained whenthe powder essentially without particles less than 45 μm was compactedat a pressure above 700 MPa, had a less shiny surface which at leastunder certain circumstances is acceptable.

EXAMPLE 2

Example 1 was repeated but as lubricant 0.5% of EBS (ethylenebisstearamide) was used and the compaction was performed with the aid ofa percussion machine (Model HYP 35-4 from Hydropulsor, Sweden)

From FIGS. 2-1 and 2-2, respectively, it can be noticed that highergreen densities and lower ejection forces were obtained with the powdercomposition according to the invention compared with the powdercomposition with the standard powder. It can also be noticed thatcomponents produced from the standard powder had deteriorated surfacesat all compaction pressures.

1. A powder composition comprising: an iron or iron-based powder whereinless than about 5% of the powder particles have a size below 45 μm; and0.1-1.0% by weight of graphite.
 2. The powder composition of claim 1,further comprising about 0.05 to 0.6% by weight of a lubricant.
 3. Thepowder composition of claim 1, wherein at least 50% of the iron-basedpowder particles have a particle size above about 106 μm.
 4. The powdercomposition of claim 3, wherein at least 60% of the iron-based powderparticles have a particle size above about 106 μm.
 5. The powdercomposition of claim 3, wherein at least 70% of the iron-based powderparticles have a particle size above about 106 μm.
 6. The composition ofclaim 3, wherein at least 50% of the iron-based powder particles have aparticle size above about 212 μm.
 7. The composition of claim 1, furthercomprising additives selected from the group consisting of alloyingelements Mn, Cu, Ni, Cr, Mo, V, Co, W, Nb, Ti, Al, P, S and B,machinability enhancing agents, hard phase materials and flow agents.